This invention relates to a new process for the recovery of adipic acid from the separated stream of by-products obtained from the oxidation of commercial mixtures of cyclohexanol/cyclohexanone with nitric acid and isolation of most of the resultant adipic acid by crystallization. At the same time, the distillation residue obtained after distillative workup of the glutaric acid left in the mother liquor from the process is put to a useful purpose.
Glutaric and succinic acid are obtained as by-products in the preparation of adipic acid by oxidation of commercial mixtures of cyclohexanol and cyclohexanone using nitric acid. Compared with adipic acid, glutaric and succinic acid are concentrated to the greatest extent in the mother liquor from the adipic acid crystallization. Part of this mother liquor is therefore discharged from the process to prevent the accumulation of by-products. After the separation of nitric acid and water, the dicarboxylic acid mixture obtained may be worked up by distillation to yield "technical glutaric acid". The distillation residue thus obtained contains mainly adipic acid, coking products, and metal catalysts such as copper or vanadium.
The adipic acid present in the dicarboxylic acid mixture (about 25 to 35% by weight) is lost as adipic acid. This adipic acid impairs the quality of the technical glutaric acid obtained, thereby impairing the usefulness of technical glutaric acid for tanning, and increases the quantity of residue from the distillative workup of glutaric acid.
Numerous processes have therefore been developed for the recovery of adipic acid. According to U.S. Pat. No. 3,790,626, adipic acid may be extracted, for example, with cyclohexanol and/or cyclohexanone. According to U.S. Pat. No. 4,146,730, glutaric and succinic acid form adducts with urea, from which adipic acid can be separated.
The conversion of glutaric and succinic acids into imides and their separation has also been described. U.S. Pat. No. 3,818,081). The reaction with alkylamine converts glutaric and succinic acid into amides, which are then separated from adipic acid (European Patent Application 33,851). U.S. Pat. No. 4,442,303 describes the esterification of dicarboxylic acids and the workup and separation of the resultant esters.
Various crystallization processes have also been described in the literature. U.S. Pat. No. 4,014,903 describes a simple cooling crystallization for the recovery of adipic acid from the separated stream of by-products. Experiments have shown, however, that the efficiency of such a process is unsatisfactory for carrying out on a technical scale because the quantity which crystallizes is too small, a large quantity of succinic acid crystallizes at the same time, heavy deposits cake to the walls of the vessels and cooling coils, and the speed of crystallization is too low because the solution frequently remains supersaturated even after about three hours.
According to U.S. Pat. No. 4,254,283, the crystallization of succinic acid that occurs at the same time is accepted and the resulting mixture of adipic acid and succinic acid is separated by distillation. The addition of sulfuric acid and water to the dicarboxylic acid mixture and the recovery of adipic acid by crystallization has also been described. "Separating and Recovering Adipic Acid", Japanese Kokai 54-115314.
It has now surprisingly been found that the disadvantages of cooled crystallization for the recovery of adipic acid can be avoided if the HNO.sub.3 concentration is reduced from about 52-60% by weight (preferably 55-60% by weight) to about 35-50% by weight (preferably to about 45% by weight) before the onset of crystallization by the addition of an aqueous adipic acid solution having an adipic acid content of from about 0.5 to about 6% by weight (preferably a saturated solution) and the adipic acid concentration is at the same time kept constant or even increased. When calculating these concentrations the above-mentioned percentage concentrations of HNO.sub.3 for the purposes of this invention, always refer to the quantity of nitric acid and water present in the solution without taking into account the organic constituents. Due to the operation of the process according to the invention, more adipic acid crystallizes because of its low solubility in dilute HNO.sub.3. The portion of adipic acid introduced with the added aqueous solution is thereby overcompensated for several times over. Crystallization proceeds more rapidly due to the higher relative super-saturation and the presence of crystallization nuclei in the added adipic acid solution. The precipitate can easily be filtered. Significantly less succinic acid crystallizes at the same time because the succinic acid concentration, in contrast to the adipic acid concentration, is lowered and because less succinic acid is precipitated due to the more rapid crystallization of adipic acid. The same applies to the reduction in the amount of deposit caked to the cooling surfaces. Both these factors enable crystallization to take place at lower temperatures and hence to increase the degree of adipic acid separation.